Electrokinetic transducing method and apparatus



Oct. 28, 1952 M. WILLIAMS 2,615,940

ELECTROKINETIC TRANSDUCING METHOD AND APPARATUS Filed Oct. 25. 1949 Fig.3

a? r if INVENTOR. Milton Williams Attorney Patented Oct. 28, 1952 Ul'lED sr TES ELECTROKINETIC TRAN SDUCING METHOD AND APPARATUS 5 Claims.

This application is in part a continuation of my pending application Ser. No. 617,911 filed Sept. 21, l945 and includes embodiments of my invention reported in my article entitled An Elec- -trokinetic Transducer published in The Review of Scientific Instruments, vol. 19, No. 10, 640-646, October, 1948.

This invention relates to electrokinetic transducing methods and apparatus.

Electrokinetic phenomena are well known in the specialized branch of science dealing with the physics and chemistry of surfaces. As full an explanation of the efiects as is known can be had by referring to any standard work in this field such as Colloid and Capillary Chemistry by H. Freundlich (translated from the 3rd German edition by H. S. Hatfield) published by E. P. Button. The subject is described in this volume under the title Capillary Electrical Phenomena on pages 239 et seq. and is briefly summarized herewith. I

There are four phenomena, generally designated as electrokinetic, involving the electrical forces set up by or applied to cause relative motion of solids and liquids in mutual contact: (1) electroosmosis: liquid is caused to move along walls of capillary tubes by the application of an M. -F.; (2) cataphore'sis or electrophoresis: solid particles move through relatively stationary liquids under the application of an E. M. F.; (3) stream or streaming potential: the E. M. F. set up by forcing liquid through capillary openings in a relatively stationary solid; (4) the Born effect: the production of a potential gradient by dropping solid particles through-liquids. Of these electrokinetic effects, my invention is concerned primarily withelectroosmosis and the stream potential. nitude of the stream potential is a direct function of, among other things, the di-electric constant of the liquid, the resistivity of the liquid and the zeta potential existingupon the association of the particular liquid with a particular solid. According to currently accepted theory, when a liquid comes in contact with the walls of a capillary opening in a solid, a double layer (perhaps of the order of molecular thickness) of electrical charges arises in the liquid, those of one sign adhering to thewall of the capillary, while at a slight distance therefrom inthe liquid are aligned an equal number of charges of pposite sign. If the double layer is considered as a condenser, the potential between its plates is designed as the zeta potential and is characteristic for any combination of liquid and solid.

amended. April 30, 1928; 370 -O. G. 757) It has been determined that the mag- This invention makes use of what might be called an electrokinetic assembly including a solid medium, usually a porous disc or plug or diaphragm, through capillary openings in which a fluid flows, and a pair of electrodes, one on either side of the solid medium, that serve to transmit either to or from the assembly an electromotive force. Hereinafter the solid medium will be designated as an electrokinetic plug and the fluid as electrokinetic fluid or simply fluid. Essentially the invention involves the use of a fixed quantity or body of fluid arranged so that at least part of it passes through the electrokinetic plug and in so moving causes a restoring force to be applied to the body of liquid, which restoring force is a monotonic increasing function of the displacement of the liquid. A restoring force is a force that operates in a direction opposite to the displacement of fluid and tends to return the displaced fluid to its original position. Apparatus constructed according to the present invention involves the provision of such structure as Will contain the body of liquid and will produce the aforedescribed restoring force.

The principle of the invention can be embodied in transducing methods or apparatus that, broadly speaking, inter-convert mechanical and electrical energy. Specific embodiments, as will be seen, are adapted particularly for either one conversion or the other. The principle is useful in the construction of measuring devices, such as devices for measuring, for example, the characteristics of pressure changes in a region, the rate of climb of aircraft, or the rate of submergence of submarines and also is useful in construct ing such transducing devices as hydrophones, phonograph pickups, seismic pickups and the like. The methods and apparatus can, of course, be used for measuring variations of any quan tities which can be translated into relative dis-v placements of the electrokinetic fluid and electrokinetic assembly.

Objects of the invention are to provide methods and apparatus for transducing using the electrokinetic eifect, wherein the method and apparatus will be reliable over wide ranges of con-' ditions.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the ac- Fig. 2 is a closed U-tube apparatus embodying the invention;

Fig. 3 is an embodiment of the invention in a rate of climb instrument;

Fig. 4 is an improved transducer adapted for observing pressure fluctuations in pipe lines and the like;

Fig. 5 is a hydrophone;

Fig. 6 is a phonograph pickup; and

Fig. '7 is a seismic pickup.

The figures will now be taken up in more detail. Fig. 1 shows a U-tube 2 containing an electrokinetic assembly that includes an electrokinetic plug 4, a flxed quantity of fluid 6, and a pair of electrodes 8 and I0 disposedoneon either side of the porous plug 4. The electrodes are connected to a potential measuring device i2 which can be of varied construction. The instrument l2 can be a galvanometer, a vacuum tube voltmeter, an oscilloscope (preferably adapted for recording), an electrometer, a bridgeapparatus. or other equipmentyfor -measuring' voltage, while preferably drawing little or no current. Hereinafter, the instrument I2, and its counterparts in other figures maybe denominated simply as a voltmeter or a meter. The=U-tube 2 can be of uniform diameter or canbe provided with-enlarged'sections or reservoirs l4 and. [5. One end. [8 of the U-tube is shown. as being. .open to the. atmosphere while the other end 20 is shown'connected to a region R, the;,characteristics of .the pressure changes, in which the apparatus is designed to measure. Under the-conditions illustrated in Fig. 1, where the levels. 22 and 24 of thefluid in the reservoirs .14 and J6 respectivelyare. identical, the pressure in the regionR is, .of course,- atmospheric. An-increase. in pressure in the region R will cause the fluidii to be displaced with a. corresponding decrease. in .the height of thefiuid in the arm 28 of the'U-tube. During thisxdisplacementqor flow of fluid a portion of the body-.of fluid. 6 necessarily passes throughthe plug 4 and" thereupon generates .a streaming .potential thatis measured by the voltmeter. l2. .The magnitude of the streaming potential is proportionalto the-rate of flow of fluid through, the plugwhich in turn depends. on. the pressure diflerence acrosstheplug. Theidifference in levels of thefluid in reservoirs l4 and; producedby the displacementof the fluid causes .a restoring force to be exerted. on thezbodyof fluid which is a monotonicincreasing-function .of thedisplacement. This force, of course,..is produced by'the actionof gravity on the fluid. A decrease in pressure in the region R causes a flow of fluid .in .theopposite direction with a rise in the level. 24 'anda. lowering of the level .22. The" dimensions. of the U-tube-and quantity of fluid are, naturally-so chosen that the anticipated changes of pressure in the region R will never. be such as to cause the fluid to leave theconflnes of the U-tube 2. If desired; a flexible diaphragm can be inserted in the U-tube- 2', forexample,'in the reservoir I6 at the level-2 3, to separate the fluidfi from the zfluid in the region R.

Fig. 2 illustrates another embodiment of the invention generally in the form of a closed U- tube 30 having an electrokinetic assembly including a porous plug 32 (through which-flows part of a body of fluid 34)"- and-a pair-of electrodes 36 and 38. The electrodes are connected to a voltmeter 4E). The .arm 42 of the U-tube is shown as being provided, preferably but not-necessarily, with an enlarged section or reservoir 44 connected to a region in which pressure-changes are to be observed. This region can be of varied character but is illustrated as a pipe line or conduit 45 hav ing valves 48 and 50 on either side of the junction between the arm 42 and the conduit 48. The other arm 52 of the U-tube terminates in a closed chamber 54. This chamber can be rigid and substantially inflexible but is illustrated as being a resilient diaphragm in the form of a metal bellows. It will be apparent that an increase of pressure in the region 45 will cause a displacement of the fluid 34 with a corresponding lowering of the level 56 and rise in the level 58. This displacement involves a flow of fluid through the plug 32 which is reflected as a voltage reading appearing on the instrument 49. In this instance the restoring force is composed of several components including the force produced by the increased head of fluid in the arm 52, the force produced by theincreased pressure of the compressed gas such as air 60 in the region above the fluid level in the chamber 54, and the force produced in the'metal of the bellows 'by'its distortion. A decrease in pressure in the region 46 will cause a. flow of fluid 34 in the opposite direction again producing a reading on the instrument 4L-As in the embodiment in Fig. 1, here, also,-a'diaphragm can be used in the reservoir 44 to sepa-. rate the fluid 34 from the material flowing-in the conduit 46. The conduit 43 is used diagrammatically to represent such conduitsas oil-or gas. pipe lines but might also be merely a tank or other container. In the apparatus shown in Fig. 2, if 40 is a voltmeter, for example, it reads zero when the pressure in the conduit 46 reaches a steady state constant value. Thus it can be said. that, in general, this embodiment :of the invention is not a pressure measuringdevice but is a device responsive to changes in pressure and-can be designed, by so proportioning its parts, to indicate over adesired range therate of change .of pressure in the conduits 45.

Fig. 3 shows another embodiment of the invention particularly adapted for measuring. rate of climb of aircraft. In this figure the U-tube of the preceding figures has been modified so that what might be considered one'arm 62-thereof surround the'other arm 64. In the drawing the arms are shown as cylinders but might be of other configuration. The electrokinetic assembly, comprising the porous plug 66 and electrodes and I0, is inserted between the two arms 62 and; The electrodes are connected to a meter. 12 to measure the voltage generated upon flow of the fluid through the plug 66. .The arms 62 and-64 are arranged concentrically for the purpos of preventing extraneous efiects due to the accumu* lation of .a difference in hydrostatic head that might be caused by tilting of the'apparatus when mounted in anaircraft. .'Preferably the cross sec.- tion of the annulus between the cylinders is the same as the interior cross-section of the'inner cylinder. To avoid disturbancescaused by temperature changes it might be desirable to enclose the apparatus in a thermally insulated container 14. The arm 64 terminates in a metal bellows or thin rubber diaphragmor the like 16. The airin the region I8 is sealed from the influence of the ambient atmospheric pressure. bythe wall-"8010f the U-tube, whereas the air in the region 82 is subject to the influence. of the atmospheric pressure-through the medium .of the metal bellows -15. When the apparatus ismounted in an aircraft the. bellows 16 expands as the aircraft climbs because the .ambient atmospheric pressure on the outside of the bellows is decreasing and the airin the re-; gion 18, originallyat sea level.atmosphericpres-:

sure, expands forcing the fluid to rise in thearm 64. The meter I2 reads only during the flow of fluid and therefore does not read when the system reaches a steady state equilibrium. It is therefore apparent that the reading of the meter I2 is an indication of the rate of climb of an aircraft. In this embodiment, also, the restoring force includes a hydrostatic head component as well as components due to a compressed gas and a distorted metal bellows.

Fig. 4 shows an improved transducer embodying the principles of this invention especially adapted for observingpressure fluctuations in pipe lines and the like. This transducer comprise a body 84, preferably of metal such as stainless steel, threaded at 8B for connection into a pipe line or other apparatus where pressure fluctuations are to be observed. A diaphragm 88 separating the fluid in the pipe line from the electrokinetic fluid is hermetically sealed into a counterbore 90, formed in the body 84, by means of a diaphragm retainer 92 threaded into the counterbore. A body insert as, made of insulating material such as Plexiglas, is held in place in the body 84 between the diaphragm 83 and a weather head v96 of neoprene or the like seated against a shoulder 98 formed in the body. A porous plug preferably in the form of a fritted glass disc I00 is cemented into a recess formed for it in the body insert. The metal diaphragm 88 serves as one of the electrodes in the electrokinetic assembly while the other electrode is a platinum wire I02 held in place by a metal contactor I00, preferably also of stainless steel, threaded at I06 into a counterbore in the insert 94. Electrical leads are connected to thebody 84 and the externally projecting portion of the contactor I04.

A resilient diaphragm in the form of a short length of rubber hose or the like I08 is slipped over the body insert 04 before. the transducer is assembled and fits snugly in a groove IIO prepared for it in the periphery of the body insert. The chamber for containing the electrokinetic fluid is constituted by the central bore I I2 in the body insert illustrated as having a smaller diameter section I I4 and a larger diameter section I I6. To allow for displacement of the electrokinetic fluid relative to the plug I00 the interior of the chamber II2 is placed in fluid communication with the interior surface of the hose I08 by holes I I8 and I20 drilled diametrically through the body insert. To insure that the chamber H2 is hermetically sealed at the end opposite the diaphragm 88 a gasket retainer I22, preferably also of Plexiglas, is forced against the body insert by the contactor I04 through a gasket I24 preferably made of neoprene or the like.

Assembly and operation of transducer of Fig. 4

To assemble the instrument the hose I00 is slipped into place on the body insert and the fritted disc I00 is cemented into place. The body insert is then placed within the body 84 and is filled with the desired electrokinetic fluid in any suitable manner such as by immersing the entire assembly in the fluid. The contactor I04 is then screwed into place and the diaphragm 88 is placed in position, care being taken that no air is trapped inside the chamber I I2. The instrument is now ready for use and when used to observe pressure fluctuations in a pipe line, for example, it is screwed into the pipe line or connecting branch thereof by means of the thread 86 so that the pressure inthe pipe line acts against the outer surfaceof thediaphragm 88. If the pressure in the pipe line increases the diaphragm 88 is bulg d inward displacing the electrokinetic fluid, the flow of which through the electrokinetic plug I00 is reflected in the voltage appearing betweenthe electrodes I02 and 88. The displacement of the fluid is permitted by the yielding of the hose I08 which thereby exerts the restoring force characteristic of all instruments embodying the principles of this invention. As the pressure in the pipe line decreases the electrokinetic fluid, under the urging of the distorted hose I08, tends to re.- turn to its original position passing back through the disc I00. As it passes through the disc on its return a voltage is, of course, again generated and appears across the electrodes. When the pressure in the pipe line reaches a steady state condition there is no displacement of the fluid and the streaming potential is zero. By properly selecting the materials to ,be used in the instrument, having due regard for such properties as mass and compliance, an instrument can be made which will respond quite accurately over wide ranges to the rate of change of pressure in the pipe line or conduit to which the instrument is applied. If it is desired to obtain a signal that is proportional to the pressure rather than to the rate of change of pressure it is necessary to integrate with respect to time. One method of doing this is by shunting the output terminals of the instrument with a condenser. A pressure-change in a pipe line constitutes what can be properly called a sonic impulse or sound, although the term "sound is more usually applied to cyclicly changing pressure especially where the frequency of variation is within the audible range. By judicious design this instrument can be made to reproduce faithfully the frequency and wave form of the pressure variations or sound occurring in the pipe line to which it is connected. Under these circumstances, where a cyclic variation of pressure occurs, the output of the instrument will be an alternating voltage. The instrument can be made to operate adequately from low frequencies approaching zero into the supersonic range. Theoretically this transducer, like almost all of the embodiments of this invention can be used to convert energy in the opposite direction. That is, instead of converting the mechanical energy of the pressure variations or sound into electrical energy it can convert electrical energy applied to the electrodes into sound or mechanical energy transmitted by the diaphragm 88. Thus if an alternating potential is impressed across the porous plug a corresponding oscillation of the fluid through the plug is produced by electro-osmosis and the instrument then operates as an electro-osmotic speaker. This invention has been so used and upon connection to the audio output of a radio receiver has been found to reproduce music and speech faithfully.

For an embodiment of the invention particularly adapted for use with sound under water reference is made to Fig. 5 which shows an arrangement known as a hydrophone including a body I26 of insulating material having a chamber I28 sealed with electrokinetic fluid therein to forma transducer with an electrokinetic assembly including a porous plug I30 and electrodes I32 and I34. The chamber I28 is sealed at one end by a thin rubber diaphragm or the like I36 and at the other end by a similar diaphragm I30. A perforated sponge rubber baille I40 is fixed to the body I26 and covers the diaphragm I38. To stabilize the body a heavy mass such as a lead ring I42 is attached to it and can serve as a convenient place for the attachment of a suspending cable or'rod I44. Electrical connectio'ns'tothe electrodes are made through-theinsulated' cable I46.

In 1 operation the hydrophone is suspended' in water, as from a ship, andsound, as from another ship-or a submarinais supposed to impinge on the diaphragm I36 movement of which forces fluid through the porous plug I 38 that is cemented or fixed in the Venturi-like throat'of the cham'- her -128. The streaming potential generated is conducted to the surface'by thecable I41. The perforated baffle I40 allows equalization of hydrostatic pressureon the diaphragms I35 and I38 but absorbs any sound which'would" otherwise impinge on the diaphragm I38. The-heavy ring I42 firmly attached'to the'body I26 provides suflicient'i'nertia' to minimize'the movement'of the body-itself under the influence of the sonioim pulse-i Fig.'6 shows'a diagrammatic reproduction-of a phonograph: pickup embodying'the present invention. The body I41 of the pickup is made of am insulating-material, suchas a plastic, and contains an electrokineticplug I49 and electrokinetic fluidl48. The chamber enclosing the electrokineti'c plug is completed by two thin metal diaphragms I50 and 152 hermetically sealing the chamber'and held in place by metal fol-lowers I54 and-I 56, respectively, threaded onto the body I41; The metal foil diaphragms also serve as electrodes in the electrokinetic assembly, Zthe streaming potential'being conducted from the'diaphragms through theirrespective followers to the leads I58 running to the phonograph amplifier. The'phonograph'needle I60 is held by a yoke I62 of insulating material, such'as plastimcemented or otherwise suitably joined'to the diaphragms. Any'suitable means, such as a lug I64, are provided for attaching the pickup'to a tone arm. Transverse vibration of the needle'transmits mechanical energy through the'yoke and diaphragms which finally appear as electrical energy traveling'along the leads'l58. Inertia of the tone arm to which the pickup is rigidly fastened minimizes vibration of'the entire pickup assembly in re sponse to vibration of the needle.

Fig. 7 is a diagrammatic illustration of a seis-" mic pickup used to detect earth vibrations such as caused by earthquakes or explosions. "A body I66 of light weight insulating material, such as a suitable plastic, 'has a chamber form'ed'therein which is closed'at one end by a diaphragm 'IEB and at theotherend'by a diaphragm Ilfiythes'e' diaphragms being of any suitable material,-pret-' erably thin rubber. In thecentral reduced portion of the chamber is located an electrokinetic plug I12 with associated electrodes-114.-Dia

phragms I68 and H8 are' held' in hermeticallyto the porous plug. The streaming'potential generated by this relative displacement is picked up by the electrodes lid and conducted-to any ap propriate recording equipment or meter.

8 though'mercuryis illustrated as serving to add to the :inertia of the electrokinetic' fluid, the device will I operate with the electrokinetic fluid alone.

In any of the illustrated embodiments of the present invention the particular elements chosen for the electrokinetic fluid and 'electrokinetic as sembly can bevariedat will dependin on structural requirements and the desired voltage outut. Generally speaking, in streaming' potential apparatus'i-t is desirable to have as high a voltage output as possible and therefore it is Preferred to use an arrangement exhibitin a high zetapotential and a' fluid having a high resistivity and' a high dielectric constant. For'electro'os motic apparatus it is usually desirable to "obtain the highest pressure diiference across theelectro kinetic plugfor any given voltage and to -ao= complish this anarrangementis chosen exhibit ing' a; highzeta potential and using a fluid having a highfdielectricconstant; A combination' of a fritted glass disc with 'redistilled water has been foundto be quite'satisfactory, particularly for use in a transducer designed for observing pres sure changes on oil-pipe lines; Howeverfother materials would be obvious to those skilled in the art. For example, discs of 'sintered quartz have been used and'other fluids have been used "as, for example, a 20% solutionof methyl alcohol where the transducer might be used in freezing weather.

In-recapitulationit can" be seen that 'certain' functional and structural characteristicsappear in'common in all the embodiments of the in vention hereinbeiore' described. Lookingto the structure, it is apparentthat' there are essentially three components viz, an electro-kinetic assembly; disposed within a container or chamber a fixed quantity of fluid'confined within the' container in such a way that p'art'of the'fiu id, at leastjoan flowback 'and' forth through the porous'plu'g of the electrokinetic assembly and some means I01 causing 'a'restoring force to be applied to the fluid so that when there is relativedisplaceme'nt'be tweenthe fluid and the eleetrokinetic assembly this force tends to restore these two-components to their origi'nal relative position: This arrangement is also'characterized by the fact-that one of :the'two first mentioned components is disposed in mechanical energy transmitting relation with what might be termed a region external to the container. The region can be a' tank or closed change and the like'. *All the embodiments of the invention are ideally reversible in the sense of being able'to'convert electrical energy either to or from -mechahical energy-.- When' the air-'- paratus'is considered as operating in the' dircc= tion of transducln'g mechanical energy into elec tricalenergy' it will-be apparent that'it canbe used to observe'changes in any quantity'which can -be convertedinto a mechanical disturbance.

For e'xample, if theregion R of Fig. 1 consistsof a closed bulb' -containin-g material whosevolume varies with temperature, that embodiment could-be designed to respond to changes in tem-' peiature. The means 'for causing a restoring forceto be applied to the liquid upon displace ment thereof relative to the eleetrokinetic assembly can take various forms. It can include a resilient diaphragm such as the bellows of Fig. 2 and 3 or the rubber membrane of Fig. 5 or the hose H39 in Fig. 4. It can include a body of air or the like such as T3 in Fig. 3 and, it should be noted, either a diaphragm or a body of compressible gas can each be used alone without the other. Alternatively or additionally the means for causing a restoring force to be applied to the fluid can include a conduit, such as the arms of a U-tube to conduct the flow of the fluid in a direction having a vertical component. The vertical component of the direction in which the arms of a U-tube extend causes the fluid upon its displacement to exhibit a difierence in head on which gravity can act to urge the fluid to return to its original position.

When the various embodiments of the invention operate in a direction to transduce mechanical energy into electrical energy either the fluid component or the electrokinetic assembly component is so disposed as to receive mechanical energy from the external region. In the embodiment shown in Figs. 1 through 6 it is the fluid component which receives the energy while the electrokinetic assembly component remains relatively stationary, whereas in Fig. 7, the fluid component remains relatively stationary and the mechanical energy is received by the body carrying the electrohinetio assembly which is thereupon, in effect, moved through or relative to the fluid. As illustrated, the diaphragms in Figs. 3 through 6 are interposed between the fluid and the external region in mechanical-energy transmitting relation. In the illustrated embodiment of Fig. 1, for example, the meniscus or free surface 24 of the fluid or the conduit 26 can be considered as placing the fluid in mechanicalenergy transmitting relation with the region B.

When the apparatus is used for transducing electrical into mechanical energy the electrokinetic assembly is usually held relatively stationary and the fluid component is caused to move relative thereto by the application of an electromotive force to the electrodes.

It can be understood from the foregoing that in any embodiment which depends upon gravity to exert the restoring force it is, of course, necessary that the displacement of the fluid have on y a component in a vertical direction. Thus, for example, in Fig. l the arms and 28 of the U-tube need not extend directly vertically as long as they extend in a direction having some vertical component.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that Within the scope of the appended claims the invention can be practiced otherwise than as specifically described.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. A container, an electrokinetic assembly within said container including a porous plug and a pair of electrodes, one on each side of said plug, a fixed quantity of fluid confined within said container in contact with said electrodes in a position to oscillate, in part at least, through the porous plug of said assembly, a surface of said fluid located in that portion of the fluid disposed to one side of said plug being in mechanicalenergy transmitting relation with. a region external to said container. and means in contact with the portion of the fluid on the other side of said plug for applying a restoring force to said fluid upon displacement of said fluid relative to said electrokinetic assembly.

2. A container, an electrokinetic assembly within said container including a porous plug and a pair of electrodes, one on each side of said plug, a fixed quantity of fluid confined within said corrtainer in contact with said electrodes in a position to oscillate, in part at least, through the porous plug of said assembly, said fluid being in mechanical-energy transmitting relation with a region external to said container, and means for causing a restoring force to be applied to said fluid upon displacement of said fluid relative to said assembly.

3. A container, a component including an electrokinetic assembly within said container, said assembly comprising a porous plug and a pair of electrodes, one on each side of said plug, a second component constituted by an electrokinetic fluid of fixed quantity confined within said container in contact with said electrodes in a position to oscillate, in part at least, through the porous plug of said assembly, one of said components being in mechanical-energy transmitting relation with a region external to said chamber, and means for causing a restoring force to be applied to said fluid upon displacement of said fluid relative to said assembly.

4. The device of claim 1 wherein a diaphragm is interposed between said fluid and said external. region and wherein said restoring force means includes a resilient diaphragm.

5. A method of transducing electrical and mechanical energy between the electrodes of an electrokinetic assembly and an external region, said assembly comprising in addition to said electrodes a porous plug disposed between said electrodes, said method comprising the steps of (1) subjecting to relative displacement said assembly and a fixed quantity of liquid confined to oscillate in part at least through the plug and in contact with said electrodes, said displacement being in mechanical-energy-transmitting relation with said external region, and (2) imposing on said fluid a restoring force Whose magnitude is a monotonic increasing function of the displacement of said fluid relative to said assembly.

MILTON WILLIAMS REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 972,029 Schwerin Oct. 4, 1910 1,757,775 Latour May 6, 1930 Re. 22,971 Burgess Feb, 10, 1948 OTHER REFERENCES Briggs (Article), Journal of Physical Chemistry, May 1928, vol. 32, p. 657, Fig. 1. 

